Currently there is no use for biome keys after opening up their related (single) biome chest in a world (that I know of). I would propose adding a new end game mimic boss that slots in between the golem and the moon lord, summoned if one of these biome keys are placed into a chest (in the same way as the existing mimic spawned from placing a Key of Light/Night into a chest) and the respective biome chest has already been opened. I'm not sure what they should drop apart from the biome chest loot, so suggestions appreciated.